The study will comprehensively assess the neuropsychological performance of hyperthyroid children and adults both during thyrotoxicosis and following establishment of euthyroid status. The hypotheses are: 1) Thyrotoxic patients will show significant neuropsychological deficits. 2) Neuropsychological deficits will improve significantly or normalize following achievement of euthyroid status. This project is just beginning accrual of subjects.